Aluminum/silicon (aluminosilicate) molecular sieves of the zeolite type are known in the art. For instance, U.S. Pat. No. 3,702,886 is one of the earliest disclosures teaching the synthesis of pentasil zeolites with high silica content.
Zeolites have framework structures consisting of a rigid three-dimensional network of SiO.sub.2 and AlO.sub.4 tetrahedra crystalline aluminosilicates which have lattice frameworks containing arrays of small voids interconnected by channels or pores. These lattices are limited in dimension to definite ranges of values. Thus, these lattice structures are capable of absorbing molecules only of certain dimensions which makes them suitable for use as molecular sieves.
Zeolite molecular sieves which are subsequently modified with the addition of phosphorus to the aluminum and silicon are also known. For instance U.S. Pat. Nos. 3,972,832; 3,911,041 and 4,665,251, all describe modification of aluminosilicate zeolites by subsequent incorporation with a phosphorus-containing compound.
In addition to aluminosilicate zeolites impregnated with phosphorous, there is also a separate class of porous synthetic silico-alumino-phosphate crystals (known in abbreviation as SAPO). U.S. Pat. No. 4,440,871 describes hydrothermal crystallization of gels consisting of silicon, aluminum and phosphorous to form microporous SAPO crystalline structures.
Such phosphates have characteristic catalytic properties depending on the species variant, and like zeolites, have different size pore openings and variations in pore geometry. Thus, the variants display specific ability to adsorb and desorb inorganic and organic molecules of characteristic sizes. However, the range of pore openings for such molecular sieves is limited to between about 2.1 and 7.4 angstroms, making them unsuitable for selective processing of molecules with effective cross sections larger than this range of pore widths.
European Patent No. 0,146,389 claims the discovery of a different type of silico-alumino-phosphate crystal which they designate as "MCM-9". MCM crystals are said to differ from their SAPO counterparts in the procedural steps of their manufacture and in their structural data, especially in their relative ion-exchange, catalytic and sorption properties of the MCM type. PCT/WIPO Publication No. WO 89/01912 claims the discovery of yet another type of silico-alumino-phosphate crystal which they designate as "VPI-5".
Derouane et al., in their article MCM-9: A Very Large Pore Silico-alumino-phosohate Molecular Sieve, in P.A. Jacobs et al., Zeolites in the Nineties, p. 119, Elsevier Publishing Co., Amsterdam, Netherlands (1989), state that they believe the two silico-alumino-phosphate molecular sieves, MCM-9 (European Patent No. 0,146,389) and VPI-5 (PCT/WIPO Publication No. WO 89/01912), have the same structure. Derouane et al. further state that the relatively wider pore sizes of the VPI-5 molecular sieves represent one phase based on aluminum-phosphate, and are limited by 18-membered rings. (Id.)
The synthesis processes for the molecular sieves disclosed in European Patent No. 0,146,389 and PCT/WIPO Publication No. WO 89/01912 necessarily include the use of structure-directing organic substances referred to as template compounds. This additional requirement for organic templates escalates production costs, especially when the scale of the synthesis is enlarged. Additionally, these structure-directing templates are organic amines or quaternary ammonium compounds which are considered environmental hazards, producing dangerous by-products and pollutants.
European Patent No. 0,146,389 and PCT/WIPO Publication No. WO 89/01912 are both limited to describing synthesis of silico-alumino-phosphate molecular sieves. Neither reference suggests the possibility of creating variations in the selective properties of such sieves by varying the starting materials or adjusting synthesis parameters. For instance, neither disclosure provides for an alumino-phosphate combination without silicon, or for optimizing the nucleation phase and the crystallization rate to control crystalline size and morphology. Such variations are not possible from the teachings of either of these disclosures.
Without such means to vary parameters, the versatility of the resulting silico-alumino-phosphate molecular sieves is limited. This limitation in versatility makes it difficult to adapt these synthesis methods to specific separation problems faced.
Our invention proposes to solve the aforementioned shortcomings in the art.